Re: [v3 PATCH 2/4] mm: move mem_cgroup_uncharge out of __page_cache_release()

[Yang Shi]

On 6/13/19 10:13 AM, Yang Shi wrote:
> On 6/13/19 4:39 AM, Kirill A. Shutemov wrote:
> > On Thu, Jun 13, 2019 at 05:56:47AM +0800, Yang Shi wrote:
> > > The later patch would make THP deferred split shrinker memcg aware, but
> > > it needs page->mem_cgroup information in THP destructor, which is 
> > > called
> > > after mem_cgroup_uncharge() now.
> > >
> > > So, move mem_cgroup_uncharge() from __page_cache_release() to compound
> > > page destructor, which is called by both THP and other compound pages
> > > except HugeTLB. And call it in __put_single_page() for single order
> > > page.
> >
> > If I read the patch correctly, it will change behaviour for pages with
> > NULL_COMPOUND_DTOR. Have you considered it? Are you sure it will not 
> > break
> > anything?

Hi Kirill,

Did this solve your concern? Any more comments on this series?

Thanks,
Yang

> So far a quick search shows NULL_COMPOUND_DTOR is not used by any type 
> of compound page. The HugeTLB code sets destructor to 
> NULL_COMPOUND_DTOR when freeing hugetlb pages via hugetlb specific 
> destructor.
>
> The prep_new_page() would call prep_compound_page() if __GFP_COMP is 
> used, which sets dtor to COMPOUND_PAGE_DTOR by default. Just hugetlb 
> and THP set their specific dtors.
>
> And, it looks __put_compound_page() doesn't check if dtor is NULL or 
> not at all.
>
> >